In order to reduce data amount of digital audio signals, many data compression methods are developed. For example, the Advanced Audio Coding (AAC) technology is matured quite quickly and is widely used. Moreover, the High Efficiency Advanced Audio Coding (HE-AAC) is emerged to pursue lower bit rates and higher audio quality. The HE-AAC technology mainly based on the AAC technology implements a spectral band replication (SBR) to obtain extremely high compression efficiency and reduce bit-rate by about 30% as well, so as to keep high audio quality at lower bit-rate.
Refer to FIG. 1A showing a functional block diagram of a conventional decoder using the HE-AAC technology. For example, an audio signal to be decoded has an original sampling frequency of fs and an audio frequency range of 0 to fa. The audio signal to be decoded is processed by an AAC decoder 10 to generate a pulse code modulation (PCM) signal with a sampling frequency fs/2. The PCM signal is transmitted to a high-efficiency module 11. A quadrature mirror filter (QMF) banks analyzer 111 of the high-efficiency module 11 demodulates and analyzes the PCM signal to generate a low-frequency band audio data having a frequency band range of 0 to fa/2 in the frequency domain and a group of coefficients representing a high-frequency band audio data having a frequency band range of fa/2 to fa. The low-frequency band audio data and the group of coefficients representing the high-frequency band audio data are transmitted to an SBR module 112 for performing SBR. After passing the low-frequency band audio data and the high-frequency band data through a combining module 113 and a QMF banks synthesizer 114, a PCM signal with a sampling frequency fs is restored.
A surround audio effect is essential in a current audio-visual playback system. A multi-channel digital audio signal capable of providing the surround effect has various formats such as the common 5.1-channel format. With respect to the 5.1-channel format, audio signals from six channels are encoded into a multi-channel digital audio signal to be stored and transmitted. After decoding the multi-channel digital audio signal into the audio signals of the six channels, with reference to FIG. 1B, the playback system applies a pair of front speakers L and R, a center speaker C, a pair of rear surround speakers RL and RR, and a bass speaker Sub to play the audio signals. For example, the front speakers L and R serve as a main channel for providing a front sound field. The center speaker C presents dialogs of a film, the rear surround speakers RL and RR provide complete sound field envelopment, and the bass speaker Sub provides a low-frequency audio output.
The HE-AAC 5.1 audio technology, combining the foregoing two technologies, is prevailing in digital video disks (DVD), digital broadcasting and digital televisions. In a conventional decoding method, the audio signal to be decoded is transmitted to a decoder as shown in FIG. 1A. The QMF banks analyzer 111, the SBR module 112, the combining module 113 and the QMF banks synthesizer 114 need to decode the audio signal six times to restore the audio signals, belonging to the six channels, to be played, such that a large amount of calculation needed by the above process inevitably impose a burden on the playback system. Therefore, one main object of the present invention is to overcome the foregoing disadvantage.